1. Field of the Invention
This invention relates to a process for producing ethylene-propylene-.alpha.-olefin terpolymers.
More particularly, it relates to a process for producing the terpolymers by copolymerizing ethylene, propylene and another .alpha.-olefin in a specified proportion of their amounts in the presence of a catalyst obtained by preliminarily activating specified catalyst components with an .alpha.-olefin, a trialkylaluminum and an aromatic ester.
The terpolymers produced by the process of the present invention are particularly suitable for processing them into cold-proof films, i.e. films for winter season or for packaging frozen goods, films which are heat-sealable at low temperature, heat-shrinkable films, highly transparent films, seats for multi-ply blow, etc.
2. Description of the Prior Art
The art of molding and processing polypropylene into films, sheets, etc. has been well known, and it has been desired to produce polypropylene which can be processed into or is suitable for molded products having superior practical physical properties required for the products, such as low-temperature impact resistance, heat-sealability by itself at relatively low temperature (referred to hereinafter as low-temperature heat-sealability), hot shrinkability exhibited when processed into stretched film (referred to hereinafter as heat-shrinkability), etc. For such a purpose, there has been effective and practically employed a process wherein a small amount of other copolymerizable component(s) is in advance mixed with a large amount of polypropylene at a suitable time and in a suitable amount in the above-mentioned production to obtain propylene copolymer and thereby improve the above-mentioned practical physical properties as compared with propylene homopolymer.
However, such an improvement in the processing physical properties of polypropylene according to the above-mentioned copolymerization process has also raised a problem. Namely, for example, in case where an ethylene-propylene (two-component system) random copolymer is obtained by a process wherein a small amount of ethylene as a copolymerizable component is in advance mixed with propylene followed by polymerization, the transparency and rigidity of molded products obtained from the resulting copolymer are remarkably reduced with the increase of the ethylene content in the copolymer; besides, when the copolymer is molded into a film which is then processed into a bag form, the end-opening property of the bag (referred to hereinafter as end-opening property) is inferior; and further, when the random copolymer is produced, the amount of soluble (atactic) polymer byproduced, which is useless as plastics, notably increases.
Thus, improvements in the above-mentioned various processing physical properties according to known two-component system random copolymerization process are restricted with respect of physical properties of the resulting polymer as well as production yield of the polymer.
Further, processes for producing terpolymers (three-component system) consisting of ethylene, propylene and an .alpha.-olefin (as illustrated in Japanese patent application laid-open No. 35487/1974), are superior in the reduction of the amount of soluble polymer related to polymer yield, as well as in the transparency, etc. as processing physical properties of polymer, but the amount of soluble polymer byproduced is still yet large presumably since usual Ziegler-Natta system catalyst is employed. Furthermore, the terpolymers according to the above-mentioned process are still yet insufficient in the low-temperature impact strength and end-opening property as practical physical properties other than the above-mentioned transparency, and in addition, they are also insufficient in the usual physical properties such as rigidity, Young's modulus, etc.
The reason for this is that consumers have been gradually demanding molded products of polypropylene, particularly films and sheets as mentioned above, having a higher level of performances or physical properties; hence evaluation of the physical properties of polypropylene employed, made by processors producing such molded products has also been becoming severer. More concretely, if a molded product of polypropylene is superior in a physical property, but inferior in another to the average level thereof, then the overall evaluation of the product is made based on the latter inferior physical property. Thus, even when improvement in one or two or more practical physical properties is aimed, notable reduction in other practical or usual physical properties cannot be permitted, and on the other hand, in order to carry out propylene polymerization smoothly and economically, it is necessary to further reduce the amount of soluble polymer byproduced than those in the prior art.
In order to solve the above-mentioned technical problems, the present inventors previously proposed a process for producing terpolymers consisting of ethylene, propylene and another .alpha.-olefin of 4 carbon atoms or more (Japanese patent application No. 3985/1975), and further proposed an improved process over the above-mentioned (Japanese patent application laid-open No. 127994/1977).
The present inventors have made strenuous studies for further improving these processes of the prior applications, and as a result have found that multi-component system copolymers having further superior physical properties to those according to the prior art can be obtained by specializing the catalyst employed and also specifying the polymerization conditions employed in the above-mentioned processes, and also the proportion of soluble polymer byproduced can be further reduced. The present invention has been attained based on this finding.